Where plants are concerned, microclimate is everything. The mulch here is key since it keeps roots cool and minimizes watering. The less you have to water a plant, the healthier it becomes. Almost any plant can stand more sun than you might think when its roots are kept cool either by mulch or by planting in a location — such as a side yard — where “its head is in the sun and its feet are in the shade.”
For example, my gardenias (Gardenia jasminoides ‘Mystery’) are squeezed between other plants — between a crape myrtle (Lagerstroemia indica) and a Chinese lantern (Abutilon hybridum), between a pride of Madeira (Echium candidum) and another Chinese lantern, between a foribunda rose and a California native spicebush (Calycanthus occidentalis). Close proximity to other plants benefits the humidity-conscious gardenia.
As long as three to four hours of sun exposure are available, even if this sun is filtered through taller surrounding shrubs or trees, ‘Mystery’ gardenias will flourish. One caveat is in order: make sure your ‘Mystery’ gardenia selection is grafted onto a Gardenia thunbergia rootstock. Otherwise, its leaves may be chronically chlorotic (yellow with green veins).